During the Second World War sentimentality reclaimed the mainstream of American popular music. Ballad recordings—slow, romantic love songs—increasingly pushed aside the hot jazz sounds of the era’s dance bands. The singers of these songs were an assortment of old and new faces—Bing Crosby, Dick Haymes, Vaughn Monroe, and Frank Sinatra. Their hit songs, recordings such as “I’ll Be Seeing You,” “Together,” “My Devotion,” and “There Are Such Things,” exchanged cynicism and irony for optimistic...